1. Field of the Invention.
This invention relates to communication systems and more particularly to modems used to transmit data over a transmission medium.
2. Description of the Prior Art.
Data modem communication systems are known which employ a relatively high data rate main channel and a relatively low data rate secondary channel, the two channels sharing a common transmission medium. For example, U.S. Pat. No. 4,273,955 discloses a data communication system which utilizes a 2400 bits per second main communication channel and a 110 bits per second auxiliary channel for telemetry information. At the receiver, high-pass and low-pass filters are used to separate the signals on the respective transmission channels.
The application of secondary channel transmission in data modem communication systems with main channel data transmission rates which are substantially higher than the aforementioned rate of 2400 bits per second, such as 14,400 bits per second, is more complex than with lower main channel data rate transmission such as 2400 bits per second, because of bandwidth limits on the transmission medium, which is normally a telephone line, and because of a higher sensitivity to disturbances at the higher data rate.